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Chapter 6
Medium Access Control
Protocols and Local Area
Networks
Part I: Medium Access Control
Part II: Local Area Networks
Chapter Overview

Broadcast Networks




All information sent to all
users
No routing
Shared media
Radio



Cellular telephony
Wireless LANs

Medium Access Control



Local Area Networks

Copper & Optical


Ethernet LANs
Cable Modem Access
To coordinate access to
shared medium
Data link layer since direct
transfer of frames



High-speed, low-cost
communications between
co-located computers
Typically based on
broadcast networks
Simple & cheap
Limited number of users
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
Part I: Medium Access Control
Multiple Access Communications
Random Access
Channelization
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
Multiple Access
Communications
Multiple Access Communications

Shared media basis for broadcast networks



Inexpensive: radio over air; copper or coaxial cable
M users communicate by broadcasting into medium
Key issue: How to share the medium?
3
2
4
1
Shared multiple
access medium
M

5
Approaches to Media Sharing
Medium sharing techniques
Static
channelization




Partition medium
Dedicated
allocation to users
Satellite
transmission
Cellular Telephone
Dynamic medium
access control
Scheduling




Random access
Polling: take turns
Request for slot in
transmission
schedule
Token ring
Wireless LANs




Loose
coordination
Send, wait, retry if
necessary
Aloha
Ethernet
Channelization: Satellite
Satellite Channel
uplink fin
downlink fout
Channelization: Cellular
uplink f1 ; downlink f2
uplink f3 ; downlink f4
Scheduling: Polling
Data from
1 from 2
Data
Poll 1
Host
computer
Inbound line
Data to M
Poll 2
Outbound line
1
2
M
3
Stations
Scheduling: Token-Passing
Ring networks
token
Data to M
token
Station that holds token transmits into ring
Random Access
Multitapped Bus
Crash!!
Transmit when ready
Transmissions can occur; need retransmission strategy
Wireless LAN
AdHoc: station-to-station
Infrastructure: stations to base station
Random access & polling
Selecting a Medium Access
Control

Applications





Scale



What type of traffic?
Voice streams? Steady traffic, low delay/jitter
Data? Short messages? Web page downloads?
Enterprise or Consumer market? Reliability, cost
How much traffic can be carried?
How many users can be supported?
Current Examples:


Design MAC to provide wireless DSL-equivalent access to
rural communities
Design MAC to provide Wireless-LAN-equivalent access to
mobile users (user in car travelling at 130 km/hr)
Delay-Bandwidth Product

Delay-bandwidth product key parameter



Coordination in sharing medium involves using
bandwidth (explicitly or implicitly)
Difficulty of coordination commensurate with
delay-bandwidth product
Simple two-station example



Station with frame to send listens to medium and
transmits if medium found idle
Station monitors medium to detect collision
If collision occurs, station that begin transmitting
earlier retransmits (propagation time is known)
Two-Station MAC Example
Two stations are trying to share a common medium
A
transmits
at t = 0
Distance d meters
tprop = d /  seconds
A
B
Case 1
A
B
Case 2
A detects
collision at
t = 2 tprop
A
B
A
B
B does not
transmit before
t = tprop & A
captures
channel
B transmits
before t = tprop
and detects
collision soon
thereafter
Efficiency of Two-Station
Example

Each frame transmission requires 2tprop of quiet time



Station B needs to be quiet tprop before and after time when
Station A transmits
R transmission bit rate
L bits/frame
L
1
1
Efficiency   max 


L  2t propR 1  2t propR / L 1  2a
L
1
MaxThroughput  Reff 

R bits/second
L / R  2t prop 1  2a
Normalized
Delay-Bandwidth
Product
a
t prop
L/R
Propagation delay
Time to transmit a frame
Typical MAC Efficiencies
Two-Station Example:
Efficiency 
1
1  2a

CSMA-CD (Ethernet) protocol:
Efficiency 
1
1  6.44a
Token-ring network

If a<<1, then
efficiency close to
100%
As a approaches
1, the efficiency
becomes low
1
Efficiency 
1  a
a΄= latency of the ring (bits)/average frame length
Typical Delay-Bandwidth Products
Distance
1m
10 Mbps
100 Mbps
1 Gbps
3.33 x 10-
3.33 x 10-
3.33 x 100
02
01
Network Type
Desk area network
100 m 3.33 x 1001 3.33 x 1002 3.33 x 1003 Local area network
10 km 3.33 x 1002 3.33 x 1003
3.33 x 1004 Metropolitan area
network
1000 km 3.33 x 1004 3.33 x 1005 3.33 x 1006 Wide area network
100000 km 3.33 x 1006 3.33 x 1007 3.33 x 1008 Global area
network


Max size Ethernet frame: 1500 bytes = 12000 bits
Long and/or fat pipes give large a
MAC protocol features








Delay-bandwidth product
Efficiency
Transfer delay
Fairness
Reliability
Capability to carry different types of traffic
Quality of service
Cost
MAC Delay Performance

Frame transfer delay



Throughput



From first bit of frame arrives at source MAC
To last bit of frame delivered at destination MAC
Actual transfer rate through the shared medium
Measured in frames/sec or bits/sec
Parameters
R bits/sec & L bits/frame
X=L/R seconds/frame
l frames/second average arrival rate
Load  = l X, rate at which “work” arrives
Maximum throughput (@100% efficiency): R/L fr/sec
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
Random Access
ALOHA


Wireless link to provide data transfer between main
campus & remote campuses of University of Hawaii
Simplest solution: just do it




A station transmits whenever it has data to transmit
If more than one frames are transmitted, they interfere with
each other (collide) and are lost
If ACK not received within timeout, then a station picks random
backoff time (to avoid repeated collision)
Station retransmits frame after backoff time
First transmission
Retransmission
Backoff period B
t
t0-X
t0
t0+X
Vulnerable
period
t0+X+2tprop
Time-out
t0+X+2tprop
+B
ALOHA Model

Definitions and assumptions
 X frame transmission time (assume constant)
 S: throughput (average # successful frame transmissions per
X seconds)
 G: load (average # transmission attempts per X sec.)
 Psuccess : probability a frame transmission is successful
S  GPsuccess
X
Prior interval
X
frame
transmission


Any transmission that
begins during
vulnerable period
leads to collision
Success if no arrivals
during 2X seconds
Abramson’s Assumption





What is probability of no arrivals in vulnerable period?
Abramson assumption: Effect of backoff algorithm is that
frame arrivals are equally likely to occur at any time interval
G is avg. # arrivals per X seconds
Divide X into n intervals of duration D=X/n
p = probability of arrival in D interval, then
G = n p since there are n intervals in X seconds
Psuccess  P[0 arrivals in 2X seconds] 
 P[0 arrivals in 2n intervals]
G 2n
2n
 2G
 (1 - p)  (1  )  e
as n  
n
Throughput of ALOHA
S  GPsuccess  Ge
2 G
e-2 = 0.184
0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0


Collisions are means
for coordinating
access
Max throughput is
max= 1/2e (18.4%)
Bimodal behavior:
Small G, S≈G
Large G, S↓0
G
4
2
1
0.
5
0.
25

0.
12
5
0
0.
00
78
12
5
0.
01
56
25
0.
03
12
5
0.
06
25
S

Collisions can
snowball and drop
throughput to zero
Slotted ALOHA




Time is slotted in X seconds slots
Stations synchronized to frame times
Stations transmit frames in first slot after frame
arrival
Backoff intervals in multiples of slots
Backoff period
B
t
kX
(k+1)X
Vulnerable
period
t0 +X+2tprop
t0 +X+2tprop+ B
Time-out
Only frames that arrive during prior X seconds collide
Throughput of Slotted ALOHA
S  GPsuccess  GP[no arrivals in X seconds]
 GP[no arrivals in n intervals]
G n
n
 G (1  p)  G (1  )  GeG
n
0.4
0.368
0.35
0.3
Ge-G
0.25
0.184
0.2
0.15
0.1
Ge-2G
0.05
G
8
4
2
1
0.5
0.25
0.125
0.0625
0.03125
0
0.01563
S
Application of Slotted Aloha
cycle
...
...
Reservation
mini-slots



X-second slot
Reservation protocol allows a large number of
stations with infrequent traffic to reserve slots to
transmit their frames in future cycles
Each cycle has mini-slots allocated for making
reservations
Stations use slotted Aloha during mini-slots to
request slots
Carrier Sensing Multiple Access
(CSMA)

A station senses the channel before it starts transmission





If busy, either wait or schedule backoff (different options)
If idle, start transmission
Vulnerable period is reduced to tprop (due to channel capture effect)
When collisions occur they involve entire frame transmission times
If tprop >X (or if a>1), no gain compared to ALOHA or slotted
ALOHA
Station A begins
transmission at
t=0
A
Station A captures
channel at t = tprop
A
CSMA Options

Transmitter behavior when busy channel is sensed



1-persistent CSMA (most greedy)
 Start transmission as soon as the channel becomes idle
 Low delay and low efficiency
Non-persistent CSMA (least greedy)
 Wait a backoff period, then sense carrier again
 High delay and high efficiency
p-persistent CSMA (adjustable greedy)
 Wait till channel becomes idle, transmit with prob. p; or
wait one mini-slot time & re-sense with probability 1-p
 Delay and efficiency can be balanced
Sensing
1-Persistent CSMA Throughput
0.6
0.53

0.5
0.4
a  0.01
0.45
0.3
0.2

0.16
a =0.1
0.1
a=1
64
32
16
8
4
2
1
0.5
0.25
0.13
0.06
0.03
0
0.02
S
G
Better than Aloha
& slotted Aloha
for small a
Worse than Aloha
for a > 1
Non-Persistent CSMA Throughput
a = 0.01
0.81
0.9

0.8
0.7
0.6
0.51
0.5
0.4
0.3
0.2
a = 0.1
0.14
0.1
a=1
64
32
16
8
4
2
1
0.5
0.25
0.13
0.06
0.03
0
0.02
S
G

Higher maximum
throughput than
1-persistent for
small a
Worse than Aloha
for a > 1
CSMA with Collision Detection
(CSMA/CD)



Monitor for collisions & abort transmission
 Stations with frames to send, first do carrier sensing
 After beginning transmissions, stations continue
listening to the medium to detect collisions
 If collisions detected, all stations involved stop
transmission, reschedule random backoff times, and
try again at scheduled times
In CSMA collisions result in wastage of X seconds
spent transmitting an entire frame
CSMA-CD reduces wastage to time to detect collision
and abort transmission
CSMA/CD reaction time
A begins to
transmit at A
t=0
B
A
B
A detects
collision at A
t= 2 tprop- 
B
B begins to
transmit at
t = tprop- ;
B detects
collision at
t = tprop
It takes 2 tprop to find out if channel has been captured
CSMA-CD Model

Assumptions
 Collisions can be detected and resolved in 2tprop
 Time slotted in 2tprop slots during contention periods
 Assume n busy stations, and each may transmit with
probability p in each contention time slot
 Once the contention period is over (a station
successfully occupies the channel), it takes X seconds
for a frame to be transmitted
 It takes tprop before the next contention period starts.
(a)
Busy
Contention
Busy
Idle
Contention
Busy
Time
CSMA/CD Throughput
Busy
Contention
Busy
Contention
Busy
Contention
Busy
Time

At maximum throughput, systems alternates between
contention periods and frame transmission times
 max

X
1
1



X  t prop  2et prop 1  2e  1a 1  2e  1Rd / L
where:
R bits/sec, L bits/frame, X=L/R seconds/frame
a = tprop/X
 meters/sec. speed of light in medium
d meters is diameter of system
2e+1 = 6.44
CSMA-CD Application: Ethernet

First Ethernet LAN standard used CSMA-CD



1-persistent Carrier Sensing
R = 10 Mbps
tprop = 51.2 microseconds



512 bits = 64 byte slot
accommodates 2.5 km + 4 repeaters
Truncated Binary Exponential Backoff

After nth collision, select backoff from {0, 1,…, 2k – 1},
where k=min(n, 10)
Throughput for Random Access
MACs
CSMA/CD
1
1-P CSMA
0.8
max
Non-P CSMA
0.6
Slotted ALOHA
0.4
ALOHA
0.2
a
0
0.01


0.1
1
For small a: CSMA-CD has best throughput
For larger a: Aloha & slotted Aloha better throughput
Carrier Sensing and Priority
Transmission


Certain applications require faster response
than others, e.g. ACK messages
Impose different interframe times




High priority traffic sense channel for time t1
Low priority traffic sense channel for time t2>t1
High priority traffic, if present, seizes channel first
This priority mechanism is used in IEEE
802.11 wireless LAN
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
Channelization
Why Channelization?

Channelization



Semi-static bandwidth allocation of portion of
shared medium to a given user
Highly efficient for constant-bit rate traffic
Preferred approach in


Cellular telephone networks
Terrestrial & satellite broadcast radio & TV
Why not Channelization?

Dynamic MAC much better at handling bursty traffic
30
M=16
25
20
M=2
0
M=1

0 .7
5
0 .6
M=4
0 .5
10
0 .4
M=8
0 .3
15
0 .2

Average transfer delay increases with number of users M
0 .1

0

Inflexible in allocation of bandwidth to users with
different requirements
Inefficient for bursty traffic
Does not scale well to large numbers of users
E[T]/X

Channelization Approaches

Frequency Division Multiple Access (FDMA)



Time Division Multiple Access (TDMA)



Frequency band allocated to users
Broadcast radio & TV, analog cellular phone
Periodic time slots allocated to users
Telephone backbone, GSM digital cellular phone
Code Division Multiple Access (CDMA)


Code allocated to users
Cellular phones, 3G cellular
Guardbands

FDMA



Frequency bands must be non-overlapping to
prevent interference
Guardbands ensure separation; form of overhead
TDMA



Stations must be synchronized to common clock
Time gaps between transmission bursts from
different stations to prevent collisions; form of
overhead
Must take into account propagation delays
CDMA Spread Spectrum Signal
Transmitter from one user
Binary
information
R1 bps
W1 Hz

R >> R1bps
W >> W1 Hz
Unique user
binary random
sequence





Radio
antenna
Digital
modulation
User information mapped into: +1 or -1 for T sec.
Multiply user information by pseudo- random binary pattern of
G “chips” of +1’s and -1’s
Resulting spread spectrum signal occupies G times more
bandwidth: W = GW1
Modulate the spread signal by sinusoid at appropriate fc
CDMA Demodulation
Signals
from all
transmitters




Signal and residual
interference


Digital
demodulation
Correlate to
user binary
random sequence
Binary
information
Recover spread spectrum signal
Synchronize to and multiply spread signal by same pseudo-random
binary pattern used at the transmitter
In absence of other transmitters & noise, we should recover the
original +1 or -1 of user information
Other transmitters using different codes appear as residual noise
Pseudorandom pattern generator

Feedback shift register with appropriate feedback
taps can be used to generate pseudorandom
sequence
g2
g0
R0
R1
g3
R2
output
g(x) = x3 + x2 + 1
The coefficients of a
primitive generator polynomial
determine the feedback taps
Time
R0 R1 R2
0
1 0 0
1
0 1 0
2
1 0 1
3
1 1 0
4
1 1 1
5
0 1 1
6
0 0 1
7
1 0 0
Sequence repeats
from here onwards
Channelization in Code Space


Each channel uses a different pseudorandom code
Codes should have low cross-correlation




If they differ in approximately half the bits the correlation
between codes is close to zero and the effect at the output
of each other’s receiver is small
As number of users increases, effect of other users
on a given receiver increases as additive noise
CDMA has gradual increase in BER due to noise as
number of users is increased
Interference between channels can be eliminated is
codes are selected so they are orthogonal and if
receivers and transmitters are synchronized

Shown in next example
Example: CDMA with 3 users


Assume three users share same medium
Users are synchronized & use different 4-bit orthogonal codes:
{-1,-1,-1,-1}, {-1, +1,-1,+1}, {-1,-1,+1,+1}, {-1,+1,+1,-1},
+1
+1
-1
User 1
x
Receiver
-1
+1
-1
User 2
User 3
+
x
-1
-1
+1
x
Shared
Medium
Sum signal is input to receiver
Channel 1: 110 -> +1+1-1 -> (-1,-1,-1,-1),(-1,-1,-1,-1),(+1,+1,+1,+1)
Channel 2: 010 -> -1+1-1 -> (+1,-1,+1,-1),(-1,+1,-1,+1),(+1,-1,+1,-1)
Channel 3: 001 -> -1-1+1 -> (+1,+1,-1,-1),(+1,+1,-1,-1),(-1,-1,+1,+1)
Sum Signal:
(+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)
Channel 1
Channel 2
Channel 3
Sum Signal
Example: Receiver for Station 2


Each receiver takes sum signal and integrates by
code sequence of desired transmitter
Integrate over T seconds to smooth out noise
Decoding signal from station 2
+
Shared
Medium
x
Integrate
over T sec
Decoding at Receiver 2
Sum Signal:
Channel 2 Sequence:
Correlator Output:
Integrated Output:
Binary Output:
(+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)
(-1,+1,-1,+1),(-1,+1,-1,+1),(-1,+1,-1,+1)
(-1,-1,+1,-3),(+1,+1,+3,-1),(-1,-1,-3,+1)
-4,
+4,
-4
0,
1,
0
Sum Signal
X
=
Channel 2
Sequence
Correlator
Output
+4
Integrator
Output
-4
-4
Channelization in Cellular
Telephone Networks

Cellular networks use frequency reuse





Band of frequencies reused in other cells that are
sufficiently far that interference is not a problem
Cellular networks provide voice connections which
is steady stream
FDMA used in AMPS
TDMA used in IS-54 and GSM
CDMA used in IS-95
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
Part II: Local Area Networks
Overview of LANs
Ethernet
802.11 Wireless LAN
LAN Bridges
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
Overview of LANs
What is a LAN?
Local area means:
 Private ownership


Short distance (~1km) between computers




low cost
very high-speed, relatively error-free communication
complex error control unnecessary
Machines are constantly moved




freedom from regulatory constraints of WANs
Keeping track of location of computers a chore
Simply give each machine a unique address
Broadcast all messages to all machines in the LAN
Need a medium access control protocol
Typical LAN Structure



Ethernet
Processor
RAM
ROM
RAM
Transmission
Medium
Network Interface
Card (NIC)
Unique MAC
“physical” address
Medium Access Control Sublayer
In IEEE 802.1, Data Link Layer divided into:
Medium Access Control Sublayer

1.




Coordinate access to medium
Connectionless frame transfer service
Machines identified by MAC/physical address
Broadcast frames with MAC addresses
Logical Link Control Sublayer
2.

Between Network layer & MAC sublayer
MAC Sub-layer
OSI
IEEE 802
Network layer
LLC
Network layer
802.2 Logical link control
Data link
layer
802.11
802.3
802.5
MAC
CSMA-CD Token Ring Wireless
LAN
Physical
layer
Various physical layers
Other
LANs
Physical
layer
Logical Link Control Layer

IEEE 802.2: LLC enhances service provided by MAC
C
A
A
Unreliable Datagram Service
Reliable frame service C
LLC
LLC
LLC
MAC
MAC
MAC
MAC
MAC
MAC
PHY
PHY
PHY
PHY
PHY
PHY
Logical Link Control Services

Type 1: Unacknowledged connectionless service


Unnumbered frame mode of HDLC
Type 2: Reliable connection-oriented service

Asynchronous balanced mode of HDLC

Type 3: Acknowledged connectionless service

Additional addressing


A workstation has a single MAC physical address
Can handle several logical connections, distinguished by
their SAP (service access points).
LLC PDU Structure
1
1 byte
1
Source
SAP Address
Destination
SAP Address
1 or 2 bytes
Control
Source SAP Address
Destination SAP Address
C/R
I/G
1
Information
7 bits
I/G = Individual or group address
C/R = Command or response frame
1
7 bits
Examples of SAP Addresses:
06 IP packet
E0 Novell IPX
FE OSI packet
AA SubNetwork Access protocol (SNAP)
Encapsulation of MAC frames
IP
Packet
LLC LLC
PDU Header
MAC
Header
IP
Data
FCS
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
Ethernet
A bit of history…








1970 ALOHAnet radio network deployed in Hawaiian islands
1973 Metcalf and Boggs invent Ethernet, random access in wired net
1979 DIX Ethernet II Standard
1985 IEEE 802.3 LAN Standard (10 Mbps)
1995 Fast Ethernet (100 Mbps)
1998 Gigabit Ethernet
2002 10 Gigabit Ethernet
Ethernet is the dominant LAN standard
Metcalf’s Sketch
IEEE 802.3 MAC: Ethernet
MAC Protocol:
 CSMA/CD
 Slot Time is the critical system parameter






upper bound on time to detect collision
upper bound on time to acquire channel
upper bound on length of frame segment generated by
collision
quantum for retransmission scheduling
max{round-trip propagation, MAC jam time}
Truncated binary exponential backoff


for retransmission n: 0 < r < 2k, where k=min(n,10)
Give up after 16 retransmissions
IEEE 802.3 Original Parameters



Transmission Rate: 10 Mbps
Min Frame: 512 bits = 64 bytes
Slot time: 512 bits/10 Mbps = 51.2 msec


51.2 msec x 2x105 km/sec =10.24 km, 1 way
5.12 km round trip distance

Max Length: 2500 meters + 4 repeaters

Each x10 increase in bit rate, must be
accompanied by x10 decrease in distance
IEEE 802.3 MAC Frame
802.3 MAC Frame
7
1
Preamble
SD
Synch





Start
frame
6
Destination
address
6
Source
address
2
Length Information Pad
4
FCS
64 - 1518 bytes
Every frame transmission begins “from scratch”
Preamble helps receivers synchronize their clocks
to transmitter clock
7 bytes of 10101010 generate a square wave
Start frame byte changes to 10101011
Receivers look for change in 10 pattern
IEEE 802.3 MAC Frame
802.3 MAC Frame
7
1
Preamble
SD
Synch
6
Destination
address
Start
frame
0
Single address
1
Group address
0
Local address
1
Global address
6
Source
address
2
Length Information Pad
4
FCS
64 - 1518 bytes
• Destination address
• single address
• group address
• broadcast = 111...111
Addresses
• local or global
• Global addresses
• first 24 bits assigned to manufacturer;
• next 24 bits assigned by manufacturer
• Cisco 00-00-0C
• 3COM 02-60-8C
IEEE 802.3 MAC Frame
802.3 MAC Frame
7
1
Preamble
SD
Synch



Start
frame
6
Destination
address
6
Source
address
2
Length Information Pad
4
FCS
64 - 1518 bytes
Length: # bytes in information field
 Max frame 1518 bytes, excluding preamble & SD
 Max information 1500 bytes: 05DC
Pad: ensures min frame of 64 bytes
FCS: CCITT-32 CRC, covers addresses, length,
information, pad fields
 NIC discards frames with improper lengths or failed CRC
DIX Ethernet II Frame Structure
Ethernet frame
7
1
Preamble
SD
Synch



Start
frame
6
Destination
address
6
Source
address
2
Type
4
Information
FCS
64 - 1518 bytes
DIX: Digital, Intel, Xerox joint Ethernet specification
Type Field: to identify protocol of PDU in
information field, e.g. IP, ARP
Framing: How does receiver know frame length?

physical layer signal, byte count, FCS
SubNetwork Address Protocol
(SNAP)





IEEE standards assume LLC always used
Higher layer protocols developed for DIX expect type field
DSAP, SSAP = AA, AA indicate SNAP PDU;
03 = Type 1 (connectionless) service
SNAP used to encapsulate Ethernet II frames
Type
ORG
3
2
SNAP PDU
LLC PDU
Information
AA AA 03
1
MAC Header
SNAP Header
1
1
FCS
IEEE 802.3 Physical Layer
Table 6.2 IEEE 802.3 10 Mbps medium alternatives
Medium
Max. Segment Length
Topology
(a)
10base5
10base2
10baseT
10baseFX
Thick coax
Thin coax
Twisted pair
Optical fiber
500 m
200 m
100 m
2 km
Bus
Bus
Star
Point-topoint link
transceivers
Thick Coax: Stiff, hard to work with
(b)
Hubs & Switches!
T connectors flaky
Ethernet Hubs & Switches
Single collision domain
(a)
     
(b)
High-Speed backplane
or interconnection fabric

Twisted Pair Cheap
Easy to work with
Reliable
Star-topology CSMA-CD



Twisted Pair Cheap
Bridging increases scalability
Separate collision domains
Full duplex operation
Fast Ethernet
Table 6.4 IEEE 802.3 100 Mbps Ethernet medium alternatives
Medium
Max. Segment
Length
Topology
100baseT4
100baseT
100baseFX
Twisted pair category 3
UTP 4 pairs
Twisted pair category 5
UTP two pairs
Optical fiber multimode
Two strands
100 m
100 m
2 km
Star
Star
Star
To preserve compatibility with 10 Mbps Ethernet:
 Same frame format, same interfaces, same protocols
 Hub topology only with twisted pair & fiber
 Bus topology & coaxial cable abandoned
 Category 3 twisted pair (ordinary telephone grade) requires 4 pairs
 Category 5 twisted pair requires 2 pairs (most popular)
 Most prevalent LAN today
Gigabit Ethernet
Table 6.3 IEEE 802.3 1 Gbps Fast Ethernet medium alternatives
Medium
Max. Segment
Length
Topology





1000baseSX
1000baseLX
1000baseCX
1000baseT
Optical fiber
multimode
Two strands
Optical fiber
single mode
Two strands
Shielded
copper cable
Twisted pair
category 5
UTP
550 m
5 km
25 m
100 m
Star
Star
Star
Star
Slot time increased to 512 bytes
Small frames need to be extended to 512 B
Frame bursting to allow stations to transmit burst of short frames
Frame structure preserved but CSMA-CD essentially abandoned
Extensive deployment in backbone of enterprise data networks and
in server farms
10 Gigabit Ethernet
Table 6.5 IEEE 802.3 10 Gbps Ethernet medium alternatives
10GbaseSR
Medium
Max. Segment
Length





10GBaseLR
10GbaseEW
Two optical
fibers
Multimode at
850 nm
Two optical fibers
Two optical fibers
Single-mode at
1310 nm
64B66B code
64B66B
Single-mode at
1550 nm
SONET
compatibility
300 m
10 km
40 km
10GbaseLX4
Two optical fibers
multimode/singlemode with four
wavelengths at
1310 nm band
8B10B code
300 m – 10 km
Frame structure preserved
CSMA-CD protocol officially abandoned
LAN PHY for local network applications
WAN PHY for wide area interconnection using SONET OC-192c
Extensive deployment in metro networks anticipated
Typical Ethernet Deployment
Server farm
Server
Server
Server
Gigabit Ethernet links
Switch/router
Server
Ethernet
switch
100 Mbps links
Hub
10 Mbps links
Department A
Gigabit Ethernet links
Ethernet
switch
100 Mbps links
Server
Hub
10 Mbps links
Department B
Switch/router
Ethernet
switch
100 Mbps links
Server
Hub
10 Mbps links
Department C
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
802.11 Wireless LAN
Wireless Data Communications

Wireless communications compelling



Easy, low-cost deployment
Mobility & roaming: Access information anywhere
Supports personal devices


Supports communicating devices




PDAs, laptops, data-cell-phones
Cameras, location devices, wireless identification
Signal strength varies in space & time
Signal can be captured by snoopers
Spectrum is limited & usually regulated
Ad Hoc Communications
C
A
B

D
Temporary association of group of stations



Within range of each other
Need to exchange information
E.g. Presentation in meeting, or distributed computer
game, or both
Infrastructure Network
Portal
Distribution System
Server
Gateway to
Portal the Internet
AP1
AP2
A1
B1
B2
A2
BSS A

BSS B
Permanent Access Points provide access to Internet
Hidden Terminal Problem
(a)
C
A
Data Frame
A transmits data frame
B
(b)
Data Frame
A

B
C senses medium,
station A is hidden from C
Data Frame
C
C transmits data frame
& collides with A at B
New MAC: CSMA with Collision Avoidance
CSMA with Collision Avoidance
(a)
B
RTS
C
A requests to send
(b)
CTS
B
A
CTS
C
B announces A ok to send
(c)
Data Frame
A sends
B
C remains quiet
IEEE 802.11 Wireless LAN

Stimulated by availability of unlicensed
spectrum






U.S. Industrial, Scientific, Medical (ISM) bands
902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz
Targeted wireless LANs @ 20 Mbps
MAC for high speed wireless LAN
Ad Hoc & Infrastructure networks
Variety of physical layers
802.11 Definitions

Basic Service Set (BSS)





Group of stations that coordinate their access
using a given instance of MAC
Located in a Basic Service Area (BSA)
Stations in BSS can communicate with each other
Distinct collocated BSS’s can coexist
Extended Service Set (ESS)



Multiple BSSs interconnected by Distribution
System (DS)
Each BSS is like a cell and stations in BSS
communicate with an Access Point (AP)
Portals attached to DS provide access to Internet
Infrastructure Network
Portal
Distribution System
Server
Gateway to
Portal the Internet
AP1
AP2
A1
B1
B2
A2
BSS A
BSS B
Distribution Services


Stations within BSS can communicate
directly with each other
DS provides distribution services:



Transfer MAC SDUs between APs in ESS
Transfer MSDUs between portals & BSSs in ESS
Transfer MSDUs between stations in same BSS


Multicast, broadcast, or stations’s preference
ESS looks like single BSS to LLC layer
Infrastructure Services

Select AP and establish association with AP





Then can send/receive frames via AP & DS
Reassociation service to move from one AP
to another AP
Dissociation service to terminate association
Authentication service to establish identity of
other stations
Privacy service to keep contents secret
IEEE 802.11 MAC

MAC sublayer responsibilities




MAC security service options


Channel access
PDU addressing, formatting, error checking
Fragmentation & reassembly of MAC SDUs
Authentication & privacy
MAC management services


Roaming within ESS
Power management
MAC Services



Contention Service: Best effort
Contention-Free Service: time-bounded transfer
MAC can alternate between Contention Periods (CPs) &
Contention-Free Periods (CFPs)
MSDUs
MSDUs
Contentionfree service
Contention
service
Point coordination
function
MAC
Distribution coordination function
(CSMA-CA)
Physical
Contention & Backoff Behavior


If channel is still idle after DIFS period, ready station
can transmit an initial MPDU
If channel becomes busy before DIFS, then station
must schedule backoff time for reattempt




Backoff period is integer # of idle contention time slots
Waiting station monitors medium & decrements backoff
timer each time an idle contention slot transpires
Station can contend when backoff timer expires
A station that completes a frame transmission is not
allowed to transmit immediately

Must first perform a backoff procedure
(a)
B
RTS
C
A requests to send
(b)
CTS
B
CTS
A
C
B announces A ok to send
(c)
Data Frame
B
A sends
(d)
C remains quiet
ACK
B
B sends ACK
ACK
Carrier Sensing in 802.11

Physical Carrier Sensing



Virtual Carrier Sensing at MAC sublayer




Analyze all detected frames
Monitor relative signal strength from other sources
Source stations informs other stations of
transmission time (in msec) for an MPDU
Carried in Duration field of RTS & CTS
Stations adjust Network Allocation Vector to
indicate when channel will become idle
Channel busy if either sensing is busy
Transmission of MPDU without
RTS/CTS
DIFS
Data
Source
SIFS
ACK
Destination
DIFS
Other
NAV
Defer Access
Wait for
Reattempt Time
Collisions, Losses & Errors

Collision Avoidance




When station senses channel busy, it waits until channel
becomes idle for DIFS period & then begins random
backoff time (in units of idle slots)
Station transmits frame when backoff timer expires
If collision occurs, recompute backoff over interval that is
twice as long
Receiving stations of error-free frames send ACK



Sending station interprets non-arrival of ACK as loss
Executes backoff and then retransmits
Receiving stations use sequence numbers to identify
duplicate frames
Frame Structure
2
2
Frame
Control
Duration/
ID



MAC header (bytes)
6
6
Address
1
Address
2
6
2
6
0-2312
4
Address
3
Sequence
control
Address
4
Frame
body
CRC
MAC Header: 30 bytes
Frame Body: 0-2312 bytes
CRC: CCITT-32 4 bytes CRC over MAC
header & frame body
Frame Control (1)
2
2
Frame
Control
Duration/
ID





MAC header (bytes)
6
6
Address
1
Address
2
2
2
4
Protocol
version
Type
Subtype
1
6
2
6
0-2312
4
Address
3
Sequence
control
Address
4
Frame
body
CRC
1
1
1
1
1
1
1
To From More
Pwr More
Retry
WEP Rsvd
DS DS frag
mgt data
Protocol version = 0
Type: Management (00), Control (01), Data (10)
Subtype within frame type
Type=00, subtype=association; Type=01,
subtype=ACK
MoreFrag=1 if another fragment of MSDU to follow
Frame Control (2)
2
2
6
6
6
2
6
0-2312
4
Frame
Control
Duration/
ID
Address
1
Address
2
Address
3
Sequence
control
Address
4
Frame
body
CRC
2
2
4
Protocol
version
Type
Subtype
To From
DS DS
Address
1
Destination
address
Destination
address
0
0
0
1
1
0
BSSID
1
1
Receiver
address
Address
2
Source
address
1
1
1
1
1
1
1
To From More
Pwr More
Retry
WEP Rsvd
DS DS frag
mgt data
Address
3
Address
4
BSSID
N/A
Data frame from station to
station within a BSS
N/A
Data frame exiting the DS
N/A
Data frame destined for the
DS
Source
address
WDS frame being distributed
from AP to AP
Source
address
Source Destination
address
address
Transmitter Destination
address
address
BSSID
1
Meaning
To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS
Frame Control (3)
2
2
Frame
Control
Duration/
ID




MAC header (bytes)
6
6
Address
1
Address
2
2
2
4
Protocol
version
Type
Subtype
1
6
2
6
0-2312
4
Address
3
Sequence
control
Address
4
Frame
body
CRC
1
1
1
1
1
1
1
To From More
Pwr More
Retry
WEP Rsvd
DS DS frag
mgt data
Retry=1 if mgmt/control frame is a retransmission
Power Management used to put station in/out of
sleep mode
More Data =1 to tell station in power-save mode
more data buffered for it at AP
WEP=1 if frame body encrypted
Physical Layers
LLC PDU
LLC
MAC
header
MAC SDU
CRC
MAC
layer
Physical layer
convergence
procedure
PLCP PLCP
preamble header

PLCP PDU
802.11 designed to


Support LLC
Operate over many physical layers
Physical medium
dependent
Physica
layer
IEEE 802.11 Physical Layer
Options
802.11
Frequency Bit Rate Modulation Scheme
Band
2.4 GHz 1-2 Mbps Frequency-Hopping Spread
Spectrum, Direct Sequence
Spread Spectrum
802.11b
2.4 GHz
11 Mbps
Complementary Code
Keying & QPSK
802.11g
2.4 GHz
54 Mbps
Orthogonal Frequency
Division Multiplexing
& CCK for backward
compatibility with 802.11b
802.11a
5-6 GHz
54 Mbps
Orthogonal Frequency
Division Multiplexing
Chapter 6
Medium Access Control
Protocols and Local Area
Networks
LAN Bridges
Hubs, Bridges & Routers

Hub: Active central element in a star topology




Twisted Pair: inexpensive, easy to insall
Simple repeater in Ethernet LANs
“Intelligent hub”: fault isolation, net configuration, statistics
Requirements that arise:
User community grows, need to interconnect hubs
Hubs are for different types of LANs
?
Hub
Two Twisted
Pairs
Two Twisted
Pairs
Station
Hub
Station
Station
Station
Station
Station
Hubs, Bridges & Routers

Interconnecting Hubs



Repeater: Signal regeneration
 All traffic appears in both LANs
Bridge: MAC address filtering
 Local traffic stays in own LAN
Routers: Internet routing
 All traffic stays in own LAN
Higher
Scalability
?
Hub
Hub
Two Twisted
Pairs
Station
Two Twisted
Pairs
Station
Station
Station
Station
Station
General Bridge Issues
Network
Network
LLC
LLC
MAC
802.3
802.3
802.5
802.5
MAC
PHY
802.3
802.3
802.5
802.5
PHY
802.3
CSMA/CD


802.5
Token Ring
Operation at data link level implies capability to
work with multiple network layers
However, must deal with



Difference in MAC formats
Difference in data rates; buffering; timers
Difference in maximum frame length
Bridges of Same Type
Network
Network
Bridge
LLC


LLC
MAC
MAC
MAC
MAC
Physical
Physical
Physical
Physical
Common case involves LANs of same type
Bridging is done at MAC level
Transparent Bridges



Interconnection of IEEE LANs with
complete transparency
Use table lookup, and
 discard frame, if source &
destination in same LAN
 forward frame, if source &
destination in different LAN
 use flooding, if destination
unknown
Use backward learning to build table
 observe source address of
arriving LANs
 handle topology changes by
removing old entries
S1
S2
S3
LAN1
Bridge
LAN2
S4
S5
S6
S1
S2
S3
LAN1
LAN2
LAN3
B1
Port 1
B2
Port 2
Address Port
S5
S4
Port 1
Port 2
Address Port
S1→S5
S1
S2
S3
S1 to S5
S1 to S5
S1 to S5
LAN1
S1 to S5
LAN2
LAN3
B1
B2
Port 1
Port 2
Address Port
S1
1
S5
S4
Port 1
Port 2
Address Port
S1
1
S3→S2
S1
S2
S3
S3S2
S3S2
S3S2
S3S2
S3S2
LAN1
LAN2
LAN3
B1
B2
Port 1
Port 2
Address Port
S1
S3
1
1
S5
S4
Port 1
Port 2
Address Port
S1
S3
1
1
S4S3
S1
S2
S3
S4
B1
Port 1
S4S3
Port 2
Address Port
S1
S3
S4
1
2
2
S3
S4S3
S4S3
LAN1
S5
S4
LAN2
LAN3
B2
Port 1
Port 2
Address Port
S1
S3
S4
1
1
2
S2S1
S1
S2
S3
S5
S4
S2S1
LAN1
LAN2
S2S1
LAN3
B1
B2
Port 1
Port 2
Address Port
S1
S3
S4
S2
1
2
2
1
Port 1
Port 2
Address Port
S1
S3
S4
1
1
2
Adaptive Learning


In a static network, tables eventually store all
addresses & learning stops
In practice, stations are added & moved all
the time


Introduce timer (minutes) to age each entry &
force it to be relearned periodically
If frame arrives on port that differs from frame
address & port in table, update immediately
Avoiding Loops
LAN1
(1)
(1)
B1
B2
(2)
B3
LAN2
B4
LAN3
B5
LAN4
Spanning Tree Algorithm
1.
Select a root bridge among all the bridges.
•
2.
Determine the root port for each bridge except the
root bridge
•
3.
root port = port with the least-cost path to the root bridge
Select a designated bridge for each LAN
•
•
4.
root bridge = the lowest bridge ID.
designated bridge = bridge has least-cost path from the
LAN to the root bridge.
designated port connects the LAN and the designated
bridge
All root ports and all designated ports are placed
into a “forwarding” state. These are the only ports
that are allowed to forward frames. The other ports
are placed into a “blocking” state.
LAN1
(1)
(1)
B1
B2
(1)
(2)
(2)
LAN2
B3
(2)
(1)
B4
(2)
LAN3
(1)
B5
(2)
LAN4
(3)
LAN1
(1)
(1)
B1
Bridge 1 selected as root bridge
B2
(1)
(2)
(2)
LAN2
B3
(2)
(1)
B4
(2)
LAN3
(1)
B5
(2)
LAN4
(3)
LAN1
(1)
R (1)
B1
B2
(2)
(2)
LAN2
R
(2)
B4
(2)
LAN3
(1)
B3
R (1)
R (1)
B5
(2)
LAN4
Root port selected for every
bridge except root port
(3)
LAN1
D (1)
R (1)
B1
B2
(2)
D (2)
LAN2
R
D (2)
B4
(2)
LAN3
(1)
B3
R (1)
R (1)
B5
(2)
LAN4
Select designated bridge
for each LAN
(3)
D
LAN1
D (1)
R (1)
B1
B2
(2)
D (2)
LAN2
R
D (2)
B4
(2)
LAN3
(1)
B3
R (1)
R (1)
B5
(2)
LAN4
All root ports & designated
ports put in forwarding state
(3)
D
Source Routing Bridges



To interconnect IEEE 802.5 token rings
Each source station determines route to
destination
Routing information inserted in frame
Routing
control
2 bytes
Route 1
Route 2
designator designator
2 bytes
2 bytes
Destination Source
Routing
address
address information
Route m
designator
2 bytes
Data
FCS
Route Discovery




To discover route to a destination each
station broadcasts a single-route broadcast
frame
Frame visits every LAN once & eventually
reaches destination
Destination sends all-routes broadcast frame
which generates all routes back to source
Source collects routes & picks best
Detailed Route Discovery





Bridges must be configured to
form a spanning tree
Source sends single-route
frame without route designator
field
Bridges in first LAN add
incoming LAN #, its bridge #,
outgoing LAN # into frame &
forwards frame
Each subsequent bridge
attaches its bridge # and
outgoing LAN #
Eventually, one single-route
frame arrives at destination





When destination receives
single-route broadcast frame it
responds with all-routes
broadcast frame with no route
designator field
Bridge at first hop inserts
incoming LAN #, its bridge #,
and outgoing LAN # and
forwards to outgoing LAN
Subsequent bridges insert
their bridge # and outgoing
LAN # and forward
Before forwarding bridge
checks to see if outgoing LAN
already in designator field
Source eventually receives all
routes to destination station
Find routes from S1 to S3
LAN 2
S1
B4
LAN 4
B1
LAN 1
S2
B5
B3
B7
B2
S3
B6
LAN 3
LAN1
B1
B3
LAN3
B4
LAN4
LAN2
LAN 5
B6
LAN5
LAN 2
S1
LAN 4
B4
B1
S2
LAN 1
B3
B5
LAN 3
B6
B7
B2
B6
LAN3
S3
B2
LAN1
B1
LAN2
B3
LAN2
B1
B4
LAN1
LAN4
LAN4
B4
LAN2
B5
LAN5
B7
B1
B4
B7
LAN 5
B4
B2
B5
B7
B1
B3
LAN4
B5
B7
LAN1
B2
B2
LAN3
B2
B5
B6
B1
B1
B4
LAN1
B3
B5
B6
B1
LAN2
LAN1
B3
B4
B2
LAN2
LAN4
B5
LAN1
B3
LAN3
B3
LAN3
B2
B3
B6
LAN1
LAN2
Virtual LAN
VLAN 1
S3
VLAN 2
S6
VLAN 3
S9
Floor n + 1
Physical
S2
S5
S8
partition
Floor n
1 2 3 4 5 6
or
7
8
switch
9
Bridge
S1
S4
S7
Floor n – 1
Logical partition
Per-Port VLANs
VLAN 1
S3
VLAN 2
S6
VLAN 3
S9
Floor n + 1
S2
S5
S8
Floor n
1 2 3 4 5 6
Bridge
7
or
8
switch
9
S1
S4
S7
Floor n – 1
Logical partition
Bridge only forwards frames to outgoing ports associated with same VLAN
Tagged VLANs


More flexible than Port-based VLANs
Insert VLAN tag after source MAC address in
each frame




VLAN protocol ID + tag
VLAN-aware bridge forwards frames to
outgoing ports according to VLAN ID
VLAN ID can be associated with a port
statically through configuration or dynamically
through bridge learning
IEEE 802.1q